1. Field of the Invention
The present invention relates to thermosetting resin compositions and use thereof. In particular, it relates to a thermosetting resin composition which contains a polyimide resin component, an amine component, an epoxy resin component, and an imidazole component as essential components and which is suitable for making circuit boards such as flexible printed circuits and build-up circuit boards, and representative uses of such a composition, such as laminates and circuit boards incorporating the composition.
2. Description of the Related Art
In recent years, in order to increase the data processing ability of electronic devices, the frequency of electric signals transmitted through circuits of wiring boards incorporated in electronic devices has been increasing. Thus, it is desirable to ensure electrical reliability of wiring (circuit) boards and to suppress a decrease in speed of transmission of electric signals and loss of electric signals in circuits that use higher-frequency electric signals.
On the circuit boards described above, insulating layers, such as a protective film for protecting the wiring board and the circuit and an interlayer insulating film for insulating between layers of a multilayered wiring board, are usually formed. Since the insulating layers such as the protective film and the interlayer insulating film are formed on the wiring board, these films are required to exhibit both insulating properties and adhesiveness to the wiring board.
In particular, when flexible printed circuits (FPCs) or build-up circuit boards are stacked to form a multilayer wiring board, the interlayer insulating films bond and fix the substrates to one another and also fix the circuit wirings since the material of the interlayer insulating films fills the gaps between the circuit wirings. Thus, the interlayer insulating films must have both excellent adhesiveness to substrates and a certain degree of flowability that can fill the gaps between the wirings of the circuit wiring. Thus, the insulating layers such as protective film and the interlayer insulating film described above are prepared by an adhesive material having adhesiveness and resin flowability.
When the above-described adhesive material is used to form insulating layers, the adhesive material preferably has, at the least, (1) properties that can impart high reliability to wiring boards in a GHz band and (2) properties that do not adversely affect the transmission of electric signals. In this manner, the data processing ability of the electronic devices can be increased by using higher-frequency electric signals.
Typical examples of known adhesive materials used in wiring boards are epoxy adhesive materials and thermoplastic polyimide adhesive materials. The epoxy adhesive materials have excellent processability, i.e., ability to bond workpieces at low temperature and low pressure and ability to fill gaps between lines of circuit wiring, and exhibit remarkable adhesiveness to workpieces. The thermoplastic polyimide adhesive materials exhibit superior heat resistance, such as low thermal expansion and high thermal decomposition temperature.
Japanese Unexamined Patent Application Publication No. 8-27430 discloses a technique that uses a film adhesive prepared by mixing a polyimide resin having a glass transition temperature in a particular range, an epoxy compound, and a compound having active hydrogen groups reactive to the epoxy compound. The film adhesive prepared by this technique is disclosed to have capacity to bond workpieces to each other at low temperature in a short time and capacity to yield reliable heat resistance at high temperature.
However, the above-described known adhesive materials do not have properties sufficient for making wiring boards suitable for higher-frequency electric signals.
To be more specific, typical epoxy adhesive materials exhibit high dielectric constant in a GHz band and high dielectric tangent after they are cured into epoxy resins. Thus, their dielectric characteristics are not satisfactory. In contrast, the thermoplastic polyimide adhesive materials exhibit low dielectric constant in a GHz band and low dielectric tangent and thus have good dielectric characteristics.
However, high temperature and high pressure are necessary to bond workpieces onto each other using thermoplastic polyimide adhesive materials. Thus, the thermoplastic polyimide adhesive materials do not have sufficient processability.
The film adhesive disclosed in Japanese Unexamined Patent Application Publication No. 8-27430 is a mixture of a polyimide resin and an epoxy compound, is capable of bonding at low temperature in a shot time, and has reliable heat resistance at high temperature. However, the specification is silent as to its ability to fill the gaps (resin flowability) in wiring circuits and its dielectric characteristics. The epoxy compound contained in this film adhesive increases low-temperature processability by decreasing the softening temperature of the film adhesive. However, the epoxy compound increases the dielectric constant and the dielectric tangent if the content thereof is large. Thus, the film adhesive disclosed in Japanese Unexamined Patent Application Publication No. 8-27430 does not achieve sufficient dielectric characteristics.
In order to increase the data processing ability of electronic devices by increasing the frequency of electric signals, the insulating layers must have sufficient adhesiveness (i), processability and handleability (ii), heat resistance (iii), and resin flowability (iv). In addition, the insulating layers are required to exhibit good dielectric characteristics (v), i.e., low dielectric constant and low dielectric tangent, in the GHz band after curing of the resin. Thus, development of an adhesive material that can produce an insulating layer that satisfies the characteristics (i) to (v) above is awaited.